A “flat box” is a standard drywall finishing tool for applying drywall compound, also called “mud,” to joints between sheets of drywall after taping the joints. Often the drywall compound is applied in three coats, called the base coat, finish coat and skim coat, using flat boxes of different sizes.
The device is the general shape of an approximately 30 degree partial cylinder. One flat side, the back plate, has a slot for expelling drywall compound. Side plates and a radius plate combine with the back plate to form an open box-like enclosure. Another flat side, called the pressure plate, completes the enclosure and is pivotably attached to the back plate so it may be pressed toward the back plate, squeezing out the drywall compound through the slot. A long handle attaches to the pressure plate so the operator may move the box along the wall and assert the pressure needed to squeeze out the compound. A flexible polymer wiper blade attached to the free end of the pressure plate provides a flexible seal that pushes the drywall compound out as the plates are pressed together.
A disadvantage of prior art flat box designs is that the edge of the wiper blade extends beyond the edge of the pressure plate, so in operation the pressure plate must be stopped short of flattening against the back plate in order to prevent damage to the wiper. Incorporation of a stop mechanism protects the wiper but leaves a volume of drywall compound inside the box when the operator stops to reload the box.
The current invention addresses this disadvantage by permitting the device to expel almost all its contents before reloading the box. Improvements shown in additional embodiments provide a variable spring tension applied to the pressure plate and independently suspended wheels for moving the box against the wall.